This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We propose to use X-ray absorption spectroscopy to study the molecular mechanisms of heavy metal detoxification by the bacterial mercury resistance system, encoded by the Tn501 mer operon. The mer system employs metal transport proteins, enzymes for transformation and reduction of mercury species to elemental volatile Hg(0), and a paradigmatic metalloregulatory system (based on MerR), all of which demonstrate specificity for Hg. In general, we are interested in elucidating the molecular mechanisms involved and in investigating the potential for engineering this system to handle other heavy metals (Cd, Au, etc.) for bioremediation. This particular program has two major targets: (a) MerR, its interactions with merO/P and/or RNA polymerase, and the role of MerD which appears to be involved in relieving MerR activation;(b) MerA (mercuric reductase) and variants that have now been shown to also catalyze the reduction of Au(III) to Au(I), with eventual disproportionation to form Au(0) colloids.